Although circadian and sleep research has made extraordinary progress in the recent years, one remaining challenge is the objective quantification of sleepiness in individuals suffering from sleep deprivation, sleep restriction, and excessive somnolence. The major goal of the present study was to apply principal component analysis to the wake electroencephalographic (EEG) spectrum in order to establish an objective measure of sleepiness. The present analysis was led by the hypothesis that in sleep-deprived individuals, the time course of self-rated sleepiness correlates with the time course score on the 2nd principal component of the EEG spectrum. The resting EEG of 15 young subjects was recorded at 2-h intervals for 32-50 h. Principal component analysis was performed on the sets of 16 single-Hz log-transformed EEG powers (1-16 Hz frequency range). The time course of self-perceived sleepiness correlated strongly with the time course of the 2nd principal component score, irrespective of derivation (frontal or occipital) and of analyzed section of the 7-min EEG record (2-min section with eyes open or any of the five 1-min sections with eyes closed). This result indicates the possibility of deriving an objective index of physiological sleepiness by applying principal component analysis to the wake EEG spectrum.

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Neurobiological mechanisms determining a possibility of parsimonious descriptions of the continuous sleep process as a sequence of a few all‐or‐nothing variables called “sleep stages” remain unknown. We tested a suggestion that stage 1 sleep (“drowsy sleep”) corresponds to a rapid decay of a wake‐promoting process and that the boundary with stages 2 separates this decay from a rapid buildup of a sleep‐promoting process. The analyzed dataset included power spectra calculated from the electroencephalographic (EEG) records obtained during attempts of 15 adults to stay permanently awake for 43–61 h and during multiple napping attempts of nine sleep‐deprived, nine sleep‐restricted, and 11 sleep‐unrestricted adults. The time courses of scores on the 1st and 2nd principal components of the EEG spectra reflected the suggested phase relationships between rapid changes in the sleep‐ and wake‐promoting processes, respectively. The 1st principal component score was permanently attenuated during wakefulness and stage 1 sleep but started to build up on the boundary with stage 2. In contrast, the 2nd principal component score started to fall down near the wake‐sleep boundary but remained unchanged across stage 2. We concluded that stage 1 sleep corresponds to the decay phase of the wake‐promoting process that precedes the buildup phase of the sleep‐promoting process during stage 2.